The present invention relates to a magnetic resonance imaging (MRI) apparatus comprising an RF coil system comprising M RF coils for detecting RF signals from a region of interest, M being an integer larger than 2, and N receiver channels for receiving and processing the detected RF signals, N being an integer larger than 1 and smaller than M.
The invention also relates to a magnetic resonance imaging (MRI) method comprising the steps of detecting RF signals from a region of interest using an RF coil system comprising M RF coils, M being an integer larger than 2, and receiving and processing the detected RF signals using N receiver channels, N being an integer larger than 1 and smaller than M.
An MRI apparatus and an MRI method of the kinds mentioned in the opening paragraphs are generally known and widely used. A magnet is used for generating a temporarily constant, uniform magnetic field through an examination region, and a radio frequency (RF) unit comprising an RF coil system is used for transmitting RF signals into the examination region to induce and manipulate magnetic resonance of dipoles disposed in the examination region and/or for receiving RF signals from the dipoles disposed in the examination region.
The article “SENSE: Sensitivity Encoding For Fast MRI”, klaas P. Pruessmann et al., Magnetic Resonance In Medicine 42:952-962 (1999) describes a concept for considerably enhancing the performance of magnetic resonance imaging by means of arrays of multiple receiver coils. In said article sensitivity encoding (SENSE) is described, which is based on the fact that receiver sensitivity generally has an encoding effect complementary to Fourier preparation by linear field gradients. By making use of an array of multiple receiver coils, scan time can be reduced and resolution can be increased. The method described takes advantage of undersampling or reduction of k-space samples and known individual coil sensitivity profiles. Since sampling is performed with a step size larger than that prescribed by the Nyquist theorem, the so-called foldover occurs during the (always necessary) Fourier transformation, so that in principle two different points from the physical image space cannot be distinguished from one another.
In practice, the anatomical region to be examined requires more coil elements and receiver channels that are provided as hardware in the MRI apparatus. An MRI apparatus produced and sold by Philips, the Gyroscan NT which is also used in practice, can handle up to six synergy/phased array coils simultaneously by using six receiver channels. But certain applications could require more receiver channels and RF coils. If there are more RF coils than receiver channels it is necessary to select coils for allocation to the available receiver channels. A larger number of receiver channels is possible in principle and would lead to shorter acquisition times or higher resolutions. But each additional receiver channel increases the costs of an MRI apparatus considerably, and each change in the number of receiver channels necessitates a redesign of the entire system because more bandwidth is required then.
It is common in the art for an RF coil system having e.g. eight RF coils to map these onto four receiver channels by combining RF signals from opposite RF coils in quadrature mode. However, such a combination is not optimal for the above mentioned SENSE method as points in space that have to be unfolded when applying the SENSE reduction are encoded equally in a single receiver channel.